The invention relates to a transmission gear, particularly one built into heavy-duty vehicles as an auxiliary gear between a main gear and the driveline leading to the drive wheels in order to double the number of gears available in the main gear. All gear steps of the main gear can thus be further transmitted to the driveline either with a lower transmission or with a higher transmission. Transmission gears of this kind usually comprise a planetary gear whose ratio of transmission will come into effect when the transmission gear is to transmit the gear steps of the main gear with a lower transmission to the driveline.
Although known transmission gears can change the overall transmission of the driveline, the drive shaft and the driven shaft of known transmission gears will always rotate in the same direction.